The present invention pertains to a method and apparatus for improving processor performance. More particularly, the present invention pertains to improving processor performance through special handling of no-op instructions.
As is known in the art, a processor includes a variety of sub-modules, each adapted to carry out specific tasks. In one known processor, these sub-modules include the following: an instruction cache; an instruction fetch unit for fetching appropriate instructions from the instruction cache; an instruction (or decoupling) buffer that holds the fetched instructions; dispersal logic that schedules the dispersal of individual instructions into separate execution pipes; execution pipes that execute the individual instructions; and an exception and retirement logic that checks for illegal operations and retires the exception-free instructions in program order. The components used for fetching instructions are often referred to as the “front end” of the processor. The components used for the execution of instructions are often referred to as the “back end” of the processor.
Programming code to be executed by the processor can sometimes be broken down into smaller components referred to as “threads.” A thread is a set of instructions whose execution achieves a given task.
In certain known architectures such as VLIW (Very Long Instruction Word) and EPIC (Explicitly Parallel Instruction Computing), instructions are packaged together into bundles. For example, in these architectures, several different templates may be used that include a plurality of instructions to be executed. Such instruction handling is provided in the IA 64 (Intel Architecture—64-bits) architecture processors manufactured by Intel Corporation, Santa Clara, Calif. Each individual instruction may be referred to as a “syllable.” In these processors two such bundles may be provided at a time from the decoupling buffer to the dispersal logic. In multithreaded processors, two or more decoupling buffers may be provided, corresponding to the number of simultaneous threads the processor can handle. Each decoupling buffer provides two bundles at a time to the dispersal logic. The two bundles would consist of six syllables, or six instructions. Depending on the type of instruction performed, each syllable is broadly categorized into one of four execution unit types: Integer (I), Memory (M), Floating-point (F), and Branch (B). The syllables from a bundle are each assigned to different execution ports. The execution ports are designed to specifically handle one or more types of the four execution unit types previously mentioned. For example, a Memory port (“M port”) can handle Memory operations (“M-ops”) or Integer operations (“I-ops”). In common processor implementations, there are a fixed number of execution ports, consisting of, for example, 4 M ports, 2 I ports, 2 F ports, and 3 B ports.
The templates that are provided for the bundles include MFI, MII, MFB, MMI, etc. For example, the MFI template includes one Memory operation, one Floating-point operation, and one Integer operation.
As is known in the art, an instruction may be provided to the execution unit that results in no significant task performance for the processor system. For example, in the Intel® x86 processor systems, a NOP (No operation) instruction causes the execution to take no action for an “instruction cycle.” An instruction cycle as used herein is a set number of processor clock cycles that are needed for the processor to execute an instruction. Because of the templates that are used, the compiler is forced to pad a bundle with a NOP instruction when no other instruction can be issued in parallel due to dependencies. Although these NOP instructions do not perform useful work, each NOP needs to be executed to maintain precise exception handling. For example, if the next integer operation is to take place after two memory operations, the MMI template may be used, but without an Integer operation. Instead, a NOP instruction is inserted in the MMI template for the Integer operation.
When a NOP instruction is one of the syllables in a bundle, the NOP is also assigned to an available execution port. NOPs are type specific (i.e. NOP.i, NOP.m, NOP.f, or NOP.b), so they must be executed on the specified port (I, M, F or B). The NOP uses an execution port, but does not utilize the task performance capabilities of the execution port. Thus, the NOP may take up execution resources that would otherwise be available for other useful operations, such as those from a different thread in an SMT implementation. As a result, in a simultaneous multi-threaded processor, all bundles may not be issued in one cycle because a NOP would have taken an execution port that a true operation would have utilized. NOPs can make up a significant percentage of programming code. Using current compilers, they can account for approximately 30% of the code density in service systems (e.g. transaction processing systems). This inherently limits the performance of a processor by having numerous NOPS, that may require little or no execution, occupying the limited execution resources.
In view of the above, there is a need for a method and apparatus for improving dispersal performance.